Warpage of injection-molded parts is a common problem in glass fiber-reinforced poly(ethylene terephthalate) (PET). Warpage is a complex phenomenon and is affected by mold design, processing parameters and post-processing handling during the cooling process. This invention addresses mainly the aspects of warpage that are affected by the resin formulation and reduces warpage by the use of blends containing PET and poly(cyclohexanedimethylene terephthalate) (PCT). Warpage is often tested for on a relative basis by molding thin rectangular plaques, in which warpage effects are pronounced. Warpage is defined for the case of plaques as the ratio of the perpendicular deviation from a plane of the corners (see formula in Example 1) to the length of the diagonal of the specimen (ASTM D1181).
Special low warpage grades of glass fiber and mica-reinforced poly(ethylene terephthalate) have been developed in the past by adding to the polymer mineral fillers such as mica, in conjunction with the glass fibers. Reduction in warpage by the use of such mineral fillers is usually explained on a mechanical basis, citing a different rheological orientation of the mica plates, as compared to the glass fibers, in the molding process, and the subsequent reinforcement by these fillers. See especially U.S. Pat. No. 4,476,274. Another factor which is often cited as contributing to lower warpage is volume contraction during crystallization of the matrix. It has now been found that a particular blend of polyesters, reinforced with glass fiber and mico results in compositions having less tendency to warp. U.S. Pat. Nos. of interest are as follows: 4,613,634, 4,536,531, 4,476,274, 4,460,731, 4,436,860, 4,393,153, 4,337,192, 4,140,670, 4,140,669, 4,124,561, 4,115,333, 4,113,692 and 3,764,576.